You can shop ethically by conventional means like searching out labels linked to ethical forums such as Ethical Fashion Forum and organisations such as the Ethical Fashion Initiative, while The Guardian has an ethical fashion directory.
However, by far the simplest way to avoid buying unethical fashion is to stay clear of large high-street labels and to favour independents instead. Almost all ethical compromises and bad ethical practices are the result of mass production. If something is only produced in limited quantities, however, it is far more likely to have been produced to a high ethical standard.
The True Cost is a documentary worth checking out. It focuses on the poor working conditions and environmental impact caused by the production of fast fashion in the developing world. One of the documentary participants, eco fashion activist Livia Firth concludes that you should only buy what you will truly love and will continue to wear for years.
As well as buying from independents, another way to shop ethically is to buy second-hand. My most treasured charity shop find was a bundle of antique Victorian aprons which I cut up and incorporated into a range of dresses.